In reading the subsequent discussion of prior art, it is to be kept in mind that the art discussed is a collection made by the inventors with the advantage of hindsight afforded by their knowledge of their own invention.
A great deal of inventive effort has been expended during the last decade in providing suitable magnetic tapes for use as information storing media. Such tape media find use in audio systems, instrumentation systems, computer systems and video systems. They usually comprise a passive backing sheet and an active, i.e. magnetic, coating on the backing sheet.
During use, these tapes are subjected to large mechanical stresses caused by quick-accelerating winding devices, abrasion from static parts associated with reading apparatus, cleaner blades, contact with guides, etc. These stresses most commonly cause an eventual wear of the magnetic coating which may manifests itself by spurious errors appearing in the information stored on the tape. Such errors can be dropout (the loss of information) or dropin (the spurious addition of information).
One of the most fruitful areas for improving magnetic tape performance has been formulation of improved binder systems for carrying electromagnetic particles which form the information-carrying component of the tapes. The binder must adhere to a backing film, for example poly(ethylene terephthalate)-type substrates, and provide an abrasion-resistant, oil resistant material of good physical properties.
Poly(vinylchloride) was one of the first polymers to be used in forming binders for use in tapes, but that material lacked a number of desirable physical properties and also tended to degrade in the presence of the iron oxide particles embedded within the polymer matrix. More recently tape-development work has been done using a large number of polymers. One of the generic types of polymers which was utilized was the polyurethane type. These polyurethane materials were recognized as contributing desirable chemical resistance (as to cleaning solvents) to the tape systems an also imparting desirable physical properties, such as abrasion resistance, to the tape.
Thus, in U.S. Pat. No. 3,149,995 to Baur, diisocyanate-based elastomer and a vinyl copolymer were described as being mixed together to form a binder system for electromagnetic particles. Similarly, in U.S. Pat. No. 3,216,846 to Henricx et al, certain types of "hindered" poly(isocyanates) were reacted with hydroxylbearing reactants to form improved binder-system components. Hendricx used a major quantity of vinyl resin in the binder system, presumably to provide flexibility.
Talley, in U.S. Pat. No. 3,144,352, also discloses the use of a polyurethane-bearing binder. His urethane-type resin is combined with a copolymer of vinylidene chloride and acrylonitrile to give a resultant binder that -- although having certain advantages -- is highly brittle. Moreover, the vinylidene chloride must be carefully stabilized against catalytic degradation caused by the iron oxide particles held within the binder.
Further attempts to incorporate the advantageous properties of polyurethane materials in tape systems are described in U.S. Pat. No. 3,320,090 to Graubert wherein a polyurethane system combines a reaction product formed of a diisocyanate, a diabasic acid and a polyol with a phenoxy resin. The resulting material, although incorporating up to 75% of urethane polymer therein, is more brittle than it has now been discovered is desired for many services.
Another disclosure of the use of polyurethane-type systems in magnetic tape binders is disclosed in U.S. Pat. No. 3,411,944 to Higashi. Higashi, in providing a tape with a high heat resistance, utilizes a triisocyanate crosslinked system which includes a silane polyol and exhibits rather low elongation and not much toughness. Toughness as defined in this disclosure as the area under the stress-strain cruve of a binder system, i.e. a material having a combination of considerable elongation and considerable tensile strength.
U.S. Pat. No. 3,384,679 to Stetz on the other hand, discloses a concept with respect to making polyurethane polymers which comprises the step of using a polyester polymeric reactant(instead of the dibasic acid described by Graubert) and allows him to attain a relatively tough elastomeric material. It is believed that it is from among materials such as those disclosed by Stetz that the polyurethanes most useful for forming magnetic tapes are selected.